1. Field of Invention
The present invention relates to high-speed data communications cables using at least two twisted pairs of wires. More particularly, it relates to cables having a central core defining plural individual pair channels.
2. Discussion of Related Art
High-speed data communications media include pairs of wire twisted together to form a balanced transmission line. Such pairs of wire are referred to as twisted pairs. One common type of conventional cable for high-speed data communications includes multiple twisted pairs that may be bundled and twisted (cabled) together to form the cable.
Modem communication cables must meet electrical performance characteristics required for transmission at high frequencies. The Telecommunications Industry Association and the Electronics Industry Association (TIA/EIA) have developed standards which specify specific categories of performance for cable impedance, attenuation, skew and crosstalk isolation. When twisted pairs are closely placed, such as in a cable, electrical energy may be transferred from one pair of a cable to another. Such energy transferred between pairs is referred to as crosstalk and is generally undesirable. The TIA/EIA have defined standards for crosstalk, including TIA/EIA-568A. The International Electrotechnical Commission (IEC) has also defined standards for data communication cable crosstalk, including ISO/IEC 11801. One high-performance standard for 100 Ω cable is ISO/IEC 11801, Category 5, another is ISO/IEC 11801 Category 6.
In conventional cable, each twisted pair of a cable has a specified distance between twists along the longitudinal direction, that distance being referred to as the pair lay. When adjacent twisted pairs have the same pair lay and/or twist direction, they tend to lie within a cable more closely spaced than when they have different pair lays and/or twist direction. Such close spacing may increase the amount of undesirable crosstalk which occurs between adjacent pairs. Therefore, in some conventional cables, each twisted pair within the cable may have a unique pair lay in order to increase the spacing between pairs and thereby to reduce the crosstalk between twisted pairs of a cable. Twist direction may also be varied.
Along with varying pair lays and twist directions, individual solid metal or woven metal pair shields are sometimes used to electromagnetically isolate pairs. Shielded cable, although exhibiting better crosstalk isolation, is more difficult and time consuming to install and terminate. Shielded conductors are generally terminated using special tools, devices and techniques adapted for the job.
One popular cable type meeting the above specifications is Unshielded Twisted Pair (UTP) cable. Because it does not include shielded conductors, UTP is preferred by installers and plant managers, as it may be easily installed and terminated. However, conventional UTP may fail to achieve superior crosstalk isolation, as required by state of the art transmission systems, even when varying pair lays are used.
Another solution to the problem of twisted pairs lying too closely together within a cable is embodied in a shielded cable manufactured by Belden Wire & Cable Company as product number 1711A. This cable includes four twisted pair media radially disposed about a “star”-shaped core. Each twisted pair nests between two fins of the “star”-shaped core, being separated from adjacent twisted pairs by the core. This helps reduce and stabilize crosstalk between the twisted pair media. However, the core adds substantial cost to the cable, as well as material which forms a potential fire hazard, as explained below, while achieving a crosstalk reduction of only about 5 dB. Additionally, the close proximity of the shield to the pairs within the cable requires substantially greater insulation thickness to maintain desired electrical characteristics. This adds more insulation material to the construction and increases cost.
In building design, many precautions are taken to resist the spread of flame and the generation of and spread of smoke throughout a building in case of an outbreak of fire. Clearly, it is desired to protect against loss of life and also to minimize the costs of a fire due to the destruction of electrical and other equipment. Therefore, wires and cables for in building installations are required to comply with the various flammability requirements of the National Electrical Code (NEC) and/or the Canadian Electrical Code (CEC).
Cables intended for installation in the air handling spaces (i.e. plenums, ducts, etc.) of buildings are specifically required by NEC or CEC to pass the flame test specified by Underwriters Laboratories Inc. (UL), UL-910, or it's Canadian Standards Association (CSA) equivalent, the FT6. The UL-910 and the FT6 represent the top of the fire rating hierarchy established by the NEC and CEC respectively. Cables possessing this rating, generically known as “plenum” or “plenum rated”, may be substituted for cables having a lower rating (i.e. CMR, CM, CMX, FT4, FT1 or their equivalents), while lower rated cables may not be used where plenum rated cable is required.
Cables conforming to NEC or CEC requirements are characterized as possessing superior resistance to ignitability, greater resistant to contribute to flame spread and generate lower levels of smoke during fires than cables having a lower fire rating. Conventional designs of data grade telecommunications cables for installation in plenum chambers have a low smoke generating jacket material, e.g. of a PVC formulation or a fluoropolymer material, surrounding a core of twisted conductor pairs, each conductor individually insulated with a fluorinated ethylene propylene (FEP) insulation layer. Cable produced as described above satisfies recognized plenum test requirements such as the “peak smoke” and “average smoke” requirements of the Underwriters Laboratories, Inc., UL910 Steiner test and/or Canadian Standards Association CSA-FT6 (Plenum Flame Test) while also achieving desired electrical performance in accordance with EIA/FIA-568A for high frequency signal transmission.
While the above-described conventional cable, including the Belden 1711 A cable due in part to their use of FEP, meets all of the above design criteria, the use of fluorinated ethylene propylene is extremely expensive and may account for up to 60% of the cost of a cable designed for plenum usage.
The solid, relatively large core of the Belden 1711A cable may also contribute a large volume of fuel to a cable fire. Forming the core of a fire resistant material, such as FEP, is very costly due to the volume of material used in the core. Solid flame retardant/smoke suppressed polyolefin may also be used in combination with FEP. However, solid flame retardant/smoke suppressed polyolefin compounds commercially available all possess dielectric properties inferior to that of FEP. In addition, they also exhibit inferior resistance to burning and generally produce more smoke than FEP under burning conditions than FEP.